Polyamide powders, which have found some significant markets particularly in the area of coatings for metal objects, must have particle sizes suited to their mode of application and their use. Thus, powders applied to metals using the fluidization-dipping technique will have particle sizes on the order of 80-300 microns, those applied by electrostatic powdering will be smaller than 60 microns, and so forth. In many cases, the particle size must remain within narrow limits.
Various techniques have been proposed to produce such powders; including, among others, cryogenic grinding of polyamide granules or hot dissolution of these same granules in a suitable solvent followed by precipitation. In both cases, the polyamide granules must be produced by polymerization of their monomers (salts of diacids and diamines, amino acids, lactams), and the resulting powder must then be selected to produce the desired particle size.
In the case of polyamides whose monomer is a lactam; i.e., essentially polyamide-12 derived from lauryllactam and polyamide-6 derived from caprolactam, original methods have been proposed for converting the lactam directly into a fine polyamide powder. These methods consist in suspending the lactam in an organic liquid (French Pat. No. 1,213,993) or dissolving it in a solvent (German Pat. No. 1,183,680) and by conducting an anionic-type polymerization directly obtaining polyamide powder, which spontaneously separates from the liquid medium as it forms.
Methods for anionic polymerization of lactams are essentially based on the use of a catalyst such as sodium or one of its compounds, such as sodium hydride or sodium methylate and an activator such as the N-carboxyanilide lactams, the isocyanates, the carbodiimides, the cyanimides, the acyllactams, the triazines, the ureas, the N-substituted imides, the esters, and the like.
Use of these processes, which initially seemed intriguing, has led to considerable frustration due to poor yields due to incomplete polymerization, untimely acceleration of the reaction leading to solidification and fouling of the sides of the reactor or the agitators, difficulties in obtaining or reproducing a given particle size, and the like deficiencies. A number of solutions have been proposed to remedy these problems (French Pat. Nos. 1,521,130, 1,601,194, 1,601,195, and 1,602,751; and German Patent Publication No. 1,942,046), such as progressive addition of the catalyst and the activator, temperature-control programming, selection of optimum solvents and others.
Despite all these improvements, none has proved successful and industrial manufacture of these powders has remained just as delicate a process requiring close attention from operating personnel.